1. Field of the Disclosure
The present invention relates to organic light emitting devices, and, more particularly, to a white organic light emitting device and a method for manufacturing the same, in which a hole transport layer is made to have an energy level higher than an energy level of an excited state of a phosphorescent light emitting layer adjacent thereto for enhancing light emitting efficiency of the hole transport layer without an additional exciton blocking layer, and a dopant content in the phosphorescent light emitting layer is adjusted for preventing color shift from taking place.
2. Discussion of the Related Art
Currently, as an information oriented age has come fully, a field of display which expresses an electric information signal visually has been developed at a high speed, and to meet this development, a variety of flat display devices which have excellent features of thin, light and low power consumption have been developed, and are replacing the present cathode ray tube CRT rapidly.
As specific examples of the flat display devices, there are a liquid crystal display devices LCD, a plasma display panel device PDP, a field emission display device FED, and an organic light emitting device OLED.
Of the flat display devices, the organic light emitting device OLED is considered as a competitive application since the organic light emitting device OLED requires no separate light source, is easy to make the device compact, and can display colors, clearly.
The organic light emitting device OLED requires an organic light emitting layer essentially, formed, in the related art, by deposition with a shadow mask.
However, since the shadow mask sags due to gravity if the shadow mask is large, is difficult to use many times, and has defects occurred in organic light emitting layer formation, alternatives have been required.
As one of many methods suggested for replacing the shadow mask, there is the white organic light emitting device.
The white organic light emitting device will be described.
The white organic light emitting device has layers between an anode and a cathode deposited without masks in formation of a light emitting diode, wherein organic films including an organic light emitting layer are deposited in a vacuum in succession with materials varied from one another.
The white organic light emitting device has many applications, such as a thin light source, a back light unit in the liquid crystal display device, or a full color display device having a color filter applied thereto, and so on.
The white organic light emitting device may have a plurality of light emitting layers having color different from one another by applying dopants of different colors to the light emitting layers respectively. However, there is limitations of composition of the dopants contained in the light emitting layers due to the properties of the dopants themselves, and, since mix of the light emitting layers is focused on production of a white light, to have a white wavelength characteristic that has a uniform peak value at a white wavelength region other than red, green, and blue wavelength regions, a color reproduction ratio becomes poor when the color filter is included to the white organic light emitting device. Moreover, since materials of the dopants are different from one another, if the white organic light emitting device is used continuously, color shift takes place.
Moreover, since energy levels of the light emitting layer and the hole transport layer similar at an interface thereof, triplet exciton passes through the interface to immigrate to the hole transport layer, dropping the light emitting efficiency in an excited state, and, if an exciton blocking layer EBL is provided for preventing this from taking place, a driving voltage rises, process steps are increased and a lifetime is shortened, imposing many obstacles in producing the white organic light emitting device of an optimum efficiency.